Method for making composite structures

ABSTRACT

A method for making a composite structure, comprising the steps of: arranging a fabric layer in a configuration constrained against outward movement and defining a cavity between opposing surfaces of the fabric layer; dispensing a predetermined amount of a self-expanding, self-curable, uncured structural foam into the cavity, the foam expanding and curing in the cavity at a molding pressure determined by the predetermined amount of the foam and thereby attaching itself to the fabric layer to form the composite structure, the molding pressure causing the expanding foam to substantially fill only interstices of an inner portion of the fabric layer, without substantially penetrating an outer portion of the fabric layer; and, freeing the cured composite structure from the constraint of the arranging step, the outer portion of the fabric layer of the composite structure being thereafter substantially completely saturable with a curable material for lamination to another structure in a subsequent processing step. The method can further comprise the step of laminating the cured composite structure to another composite structure by saturating the outer portion of the fabric layer of the cured composite structure with a curable resin.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of application Ser. No.08/651,621, filed May 21, 1996, now Pat. No. 5,800,749. Application Ser.No. 08/651,621 is a continuation of application Ser. No. 08/345,899,filed Nov. 28, 1994, now abandoned. Application Ser. No. 08/345,899 is acontinuation-in-part of application Ser. No. 08/181,321, filed Jan. 14,1994, now U.S. Pat. No. 5,429,066.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a composite structure and method of makingcomposite structures. More particularly, the invention relates tocomposite structures which are especially adapted for simplifyingfabrication of a number of articles such spas, baths, Jacuzzis, showerenclosures and the like. The composite structure in accordance with theinvention is especially adapted for use in lower stress environmentsthan such composite structure technology has generally been applied toin the past.

2. Description of Related Art

In the past, a number of structural composite articles have beendeveloped using a variety of different technologies. One prior arttechnique involves interweaving yarns of polyester fibers with yarns ofglass fibers to form a composite fabric. The fabric is then fittedwithin a mold and soaked with resin. The formed fabric and resincomposition is then allowed to harden within the mold. While providing agenerally rigid structure, such a technique includes a number oflimitations, among them the inability to produce a variety of differentshapes of substantial bulk or complex dimensions.

Another prior art technique involves manufacturing a light weightenergy-absorbing structure made up of a laminate including a pluralityof fabrics. The fabrics are pre-impregnated with a thermosetting resinand abutted against a core of cellular foam which has had a bondingmaterial applied at the interface between the impregnated fabric and thefoam. Heat is applied to cure the resin and the bonding material.However, when bonding material is used, it is often the case in hostileenvironments that the bonding material breaks down, an undesirableresult when such structures are used in high load or stressapplications.

Yet still another prior art technique involves the manufacture of athermo-formable composite article comprised of a core layer made of asynthetic resin foam. A pair of adhesive layers are placed in contactwith the opposite upper and lower surfaces of the core layer and a pairof fabric layers are superimposed on the opposite upper and lowersurfaces of the core layer. The fabric layers are bonded to the corelayer by the adhesive layers. At least one of the fabric layers is madeof a non-woven fabric which has been impregnated with a thermoplastic orthermosetting resin. This technique also involves the disadvantagesnoted above with respect to the use of adhesives.

In boat building, especially glass fiber boats, hulls are typicallyreinforced by a grid arrangement of structural members known as"stringers" and "bulkheads," hereafter collectively referred to as"stringers." Present boat-building technology typically employs plywoodstringers, although a small percentage uses glass fiber/foam stringers.The plywood stringer system involves several process stages. The plywoodis first cut to shape by a cutter or by hand with the use of templates.The stringer system which is commonly made up of interlocking stringersand bulkheads is assembled outside of the boat, with interlocking piecesbeing held with staples. Once the system is assembled, it is then placedin a boat as a module and laminated into place using a combination ofreinforcing fabrics.

Glass fiber stringers are installed in a slightly different manner fromplywood stringers. One way of installing such stringers is to cut blocksof foam into the shape of the stringers. The stringer shaped foam isthen bonded into the hull and covered with glass fiber. Another way isthat the glass fiber shell of the stringer is first laminated using amale mold. The stringer is then ground to fit into the boat and once thestringer has been ground to fit, it is spaced off of the bottom of theboat to avoid cracking problems. The stringer is then laminated intoplace and holes are drilled into the stringer and the stringer is filledwith a two-part urethane foam.

Future boat construction is expected to use closed molding technologies.The reason for this is 1) to control noxious vapor emissions into theenvironment, 2) to reduce labor costs and 3) to create structurallysuperior constructions. Such future technology will generally requireuse of stringer systems made of composite structures compatible withsuch closed moldings. Conventional stringer systems are not well suitedto such technologies since they will need to be installed after the hullhas been laminated using an open mold. Wood stringers would be difficultto incorporate into a closed mold due to height and width constraints.Further, prepositioning of glass fiber reinforcement normally handlaminated over the wood or foam stringer system would be costly andcreate quality control problems. Conventional glass fiber stringers alsopose problems due to the void in the stringer structures. Specifically,since foam is injected into such stringer after installation, it becomesdifficult to keep the resin from filling the inside of the stringer.

In accordance with the invention, these and other problems with theprior art are avoided by providing a composite structure and method ofmaking such structure which avoids the use of adhesive and lends itselfto a variety of shapes, structures and applications. In one specificaspect, the composite articles manufactured in accordance with themethod are used as stringers and bulkheads in boat manufacturingtechnology greatly simplifying the manufacture of boats, and permittingtheir use in closed mold environments.

SUMMARY OF THE INVENTION

The construction methods discussed above require reinforced fabriclayers for adequate strength of the final composite structure. However,composite structures can be used in applications requiring less strengththan frames for boats and the like. One example is as reinforcingmembers for spas, baths, Jacuzzis, shower enclosures and the like. Thecomposite structure in accordance with the invention is especiallyadapted for use in lower stress environments than such compositestructure technology has generally been applied to in the past. Suchstructures need to be stiffened and reinforced, but do not require thesame level of reinforcement as, for example, boats. In this regard, ithas been realized that composite structures intended for lighter loadscan be efficiently and inexpensively manufactured according to thetechniques developed for high strength applications as taught in U.S.Pat. No. 5,429,066, but without the need for reinforcing the fabriclayer, typically a non-woven fabric layer, provided that penetration ofthe curing foam is controlled so as to leave an outer portion of thefabric layer of the cured composite structure substantially free ofcured resin.

A method for making such a composite structure in accordance with aninventive arrangement, comprises the steps of: arranging a fabric layerin a configuration constrained against outward movement and defining acavity between opposing surfaces of the fabric layer; dispensing apredetermined amount of a self-expanding, self-curable, uncuredstructural foam into the cavity, the foam expanding and curing in thecavity at a molding pressure determined by the predetermined amount ofthe foam and thereby attaching itself to the fabric layer to form thecomposite structure, the molding pressure causing the expanding foam tosubstantially fill only interstices of an inner portion of the fabriclayer, without substantially penetrating an outer portion of the fabriclayer; and, freeing the cured composite structure from the constraint ofthe arranging step, the outer portion of the fabric layer of thecomposite structure being thereafter substantially completely saturablewith a curable material for lamination to another structure in asubsequent processing step.

The method can further comprise the step of laminating the curedcomposite structure to another composite structure by saturating theouter portion of the fabric layer of the cured composite structure witha curable resin.

The method can still further comprise the step of arranging a fabriccomposed of continuous thermoplastic fiber needle, punched together as afelt-like fiber layer in said configuration constrained against outwardmovement and defining said cavity.

The method can yet further comprise the step of selecting said fabriclayer from one of: a polyester staple mat; a glass fiber mat; a organicfiber mat; and, an inorganic fiber mat.

The method yet further comprise the step of dispensing a predeterminedamount of a two-part, self-expanding, self-curing urethane foam.

The method can yet further comprise the step of experimentallydetermining an amount of said dispensed foam to be sufficient to causesaid foam to penetrate into interstices of said inner portion of saidfabric layer as said foam expands and cures, without penetrating intosaid outer portion of said fabric layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial, side cross-sectional view of a composite structurein accordance with the invention;

FIG. 2 is a block diagram showing the sequence of steps of making thecomposite structure in accordance with the method of the invention;

FIG. 3 is a block diagram showing a sequence of steps illustrating themethod of manufacturing boats using the composite structure inaccordance with the invention as stringers for the hull of the boat;

FIG. 4 is an end, cross-sectional view of a typical stringerconstruction made according to the method of the invention; and

FIG. 5 is an end, cross-sectional view of an alternative embodiment tothe stringer shown in FIG. 4, which omits the reinforcing fabric layer.

FIG. 6 is a block diagram showing a pressurized foam injection systemhaving a time variable dispensing control system.

FIG. 7 is a block diagram showing a sequence of steps of making thecomposite structure in accordance with an alternative method of theinvention.

FIG. 8 is a block diagram showing a sequence of steps of making thecomposite structure in accordance with a second alternative method ofthe invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a typical construction for a composite structure 11in accordance with the invention. The structure 11 is made up of areinforcing fiber layer 13, preferably a directional reinforcing fiberlayer of organic or inorganic structural reinforcing fibers such asglass fiber, carbon fibers, aramid fibers such as is available under thename Kevlar, linear polyethylene or polypropylene fibers such as isavailable under the name Spectra, or polyester fibers. By reinforcingfiber is meant a fiber which when added to a composite material enhancesthe structural properties of the material. The fibers can be randomlyoriented, or preferentially, they can be oriented in one or moredirections. While a number of specific types of materials have beengiven for use as the reinforcing fiber layer, it will be appreciated bythose of ordinary skill in the art that other equivalent-typereinforcing fiber layers can be employed in the practice of theinvention.

The fiber layer 13 is attached to a fabric layer 15 which has a largenumber of interstices or pores 19. The fabric layer 15 is preferably anon-woven fabric composed of continuous thermoplastic fiber, needlepunched together to yield a felt like fabric. One example of such afabric is Trevira Spunbond, which is commercially available from HoechstCelanese Corporation. However, the invention is not limited in thisregard and the fabric layer 15 may be comprised of other materials,including polyester staple mat and glass fiber mat, as well as otherorganic and inorganic fiber mats and fabrics.

Preferably, the reinforcing fiber layer 13 and fabric layer 15 aresecured to each other mechanically. Such mechanical attachment istypically done by conventional stitching 17. Other means of attachment,as noted previously, include alternatives such as needle punching,stapling or buttons. In the case of certain non-hostile environmentapplications even adhesives can be used as discussed previously.

A structural foam 21 is attached to the fabric layer 15 by havingpenetrated into the interstices 19 at penetration region 23 of the foam21 during the manufacture of the structure 11. Preferably the foam is aself-expanding, self-curing urethane foam which has been caused toexpand at region 23 into the interstices 19 of fabric layer 15 by havingbeen filled into a mold in an amount sufficient to cause pressure as aresult of expansion of the foam 21 to penetrate at region 23 into theinterstices 19 of the fabric layer 15.

The interaction between the foam 21 and fabric layer 15 provides anumber of mechanical advantages over other structures. Morespecifically, the low density urethane foams used in the practice of theinvention typically have tensile strengths of about 50 pounds per squareinch (psi) with an elongation of under 7%. When such a foam is expandedinto a non-woven fabric, the mechanical properties of the fabric/foamsystem increases significantly. In the case where the fabric used is acontinuous filament needle punched thermoplastic polyester fabric suchas Trevira Spunbond, the mechanical properties of the Trevira/foamsystem increases to around 2,000 to 3,000 psi, with an elongation ofover 20%. All of these non-woven fibers are typically non-woven organicor inorganic fiber mat, for example, polyester, fabrics of conventionalconstruction.

The method of making the composite structure in accordance with theinvention is illustrated in FIG. 2. In a first step 25 the fabric layer15 is attached to the reinforcing fabric layer 13. Preferably suchattachment is done by stitching. Alternatively, as noted previously, thefabrics can be secured to each other by means of needle punching,staples or buttons or other mechanical technologies sufficient to secureone fabric to the other. After attaching the two layers of fabricstogether, the attached layers are laid in a mold at a step 27 with thelayer 15 facing the inside of the mold.

At step 29 a self-expanding, self-curing structural foam isprepared/mixed. Typically, such a self-rising, self-curing foam is aurethane foam commercially available from BASF, MOBAY, PPG and istypically an MDI-based rigid polyurethane foam(methylene-diphenyl-methane diisocyanate) using "hydrogenatedchlorofluorocarbons" (HCFC), water and/or CO₂ as a blowing agent. Themold is then filled with the self-expanding and self-curing structuralfoam at step 31 in an amount sufficient to allow for expansion of thefoam after the mold is closed. This generates enough pressure so thatthe foam penetrates into the interstices 19 of the fabric layer 15. Onaverage, enough foam is added to generate pressures of about 5-18"pounds per square inch" (psi), at a minimum 5 psi, and at an optimumpressure of 10 psi. Different mold pressure may be used, however, andupper pressure limits are dictated primarily by mold strength. For agiven mold pressure, foam amounts and thickness of the non-woven layercan be adjusted so that mold completely fills with the foam, but thefoam does not penetrate through the reinforcing layer. The mold isclosed at step 33 and the foam is allowed to cure and expand therein. Atstep 35 the formed article is then removed after curing.

FIG. 6 shows a foam dispensing system for a two-part, self-expanding,self-curing foam which can be utilized for an alternative methodaccording to the present invention as shown in FIGS. 7 and 8. Thefoam-dispensing system 56 includes storage tanks 57, 59 for separatelystoring the constituent materials which comprise the self-expanding,self-curing foam. The constituent foam materials are forced underpressure through hoses 61, 63 to nozzle 65. Nozzle 65 is provided formixing the constituent materials necessary to form the self-expanding,self-curing foam, and dispensing said foam in a controlled manner.Nozzle 65 includes one or more valves to control dispensing of foam,which valves may be controlled by a timing device 67. Timing device 67may be pre-set to cause the valves associated with nozzle 65 to open fora pre-determined duration, and thereby dispense a precisely-determinedmass of self-expanding, self-curing foam.

An alternative method of making the composite structure in accordancewith the present invention is illustrated in FIG. 7. In a first step 67,the fabric layer 15 is attached to the reinforcing fabric layer 13, aspreviously described. After attaching the two layers of fabric together,the attached layers are laid in a mold at a step 69 with the fabriclayer 15 facing the inside of the mold.

At step 71, the foam-dispensing system is prepared to dispenseself-expanding, self-curing structural foam. In this step, timing device67 is preset to open the valves in nozzle 65 for a predetermineddispensing time, so that the foam-dispensing system releases apredetermined mass of foam upon command. At step 73, the foam-dispensingsystem is caused to fill the mold with self-expanding, self-curingstructural foam. If the timing device for the foam-dispensing system hasbeen preset, the mold will be filled with a precise amount of foamcorresponding to the dispensing time set on the timing device 67 in step71. At step 75, the mold is closed and the foam allowed to cure andexpand. Finally, at step 77, the formed article is removed after curing.

A second alternative method of making the composite structure inaccordance with the invention is illustrated in FIG. 8. Steps 79, 81 and83 in FIG. 8 are identical to steps 67, 69 and 71, respectively, in FIG.7, except that the mold is provided with a foam injection port to allowfoam into the interior of the mold. The foam injection port may becomprised of any suitable opening formed in a wall of the mold or amechanical fitting capable of allowing the passage of foam into themold. If the injection port is comprised of a simple opening, it ispreferably located on an upper area of the mold, such as the mold lid,to prevent foam from escaping as the mold is filled. An opening is cutin the attached layers where they cover the injection port to allow thepassage of foam through the attached layers.

At step 85, rather than filling an open mold with self-expanding foam,the attached fabric layers are positioned on the mold lid, so that thewoven fabric layer faces the lid, and the non-woven layer faces insideof the mold when the lid is closed. After the lid has been closed instep 85, the foam-dispensing system is activated in step 87 so thatself-expanding, self-curing structural foam is dispensed into the moldthrough the injection port. If the timing device for the foam-dispensingsystem has been pre-set, the mold will be filled with a precise amountof foam corresponding to the dispensing time set on the timing device67. At step 89, the foam is allowed to cure and expand. Finally, at step91, the cured structural pre-form is removed from the mold.

With regard to the method of FIGS. 7 and 8, the precise mass of foamdispensed into the mold and the pre-set dispensing time set on thetiming device for controlling said volume, are preferably determinedexperimentally to achieve penetration of said foam into the non-wovenlayer, but not into the woven layer of the attached fabric layers. Themold structure must be designed to withstand the pressure generated bythe expanding foam. Typically, foam pressure may range from 5 to 18 psi.

In the method according to FIGS. 2, 7 and 8, one or more overflow portscan be provided on a portion of the mold to release excess foam, andlimit foam pressure within the mold to a range of between 5 to 18 psi.The overflow port is preferably located on an upper area of the moldsuch as the mold lid. This placement prevents foam from escaping whenthe mold is initially filled, and permits foam to escape only after ithas begun to expand. The overflow port can be comprised of a simpleopening cut in a wall of the mold. Alternatively, a suitable fitting maybe used which passes through a mold wall, and allows excess foam toescape the interior of the mold. If such overflow ports are used, a holeis cut in the attached fabric layers where they pass over the overflowport to allow the foam to escape the interior of the mold as it expandsunder pressure. In the method according to FIG. 8, the foam injectionport can also serve as an overflow port. The size and number of theoverflow ports are preferably determined experimentally to ensure thatwhen excess foam has been added to a mold, the foam does not penetratethrough the reinforced fabric layer.

The articles manufactured in accordance with the method of the inventioncan be molded in a variety of different shapes and applied in a numberof different technologies. One specific application for the articles isin the boat building industry wherein the articles can be shaped asstringers to replace conventional wood stringers or glass fiberstringers used in the manufacture of glass fiber boats. The use ofstringers in accordance with the practice of the invention results in amodular approach to boat manufacturing that saves the boat builder timeand money, particularly for small boats. More specifically, conventionalstringer systems take approximately six man hours to install. Stringersmanufactured in accordance with the method of the invention requireapproximately twenty to forty man minutes to install on a cured hull. Ifthe stringer system of the invention is installed and wet out with resinwhile the hull laminate resin is still "wet," i.e., uncured, moldingtime can also be significantly reduced. Thus, the process reductionresults in substantial cost savings and decreased mold cycle times.

The stringers in accordance with the structure of the invention areespecially adapted for use in boat construction. Typically, open-moldedboat construction begins with a gel coat being first sprayed into amold. Once the gel coat has cured, a skin coat consisting of choppedstrand mat or chopped roving, woven roving or stitch bonded glass fiberis laid on the cured gel coat layer. This will result in the structurallayer of the boat. Typically, such a layer is then impregnated with aresin and allowed to cure to form the structural layer, and thestringers are laid therein after curing.

In accordance with the method of the invention as illustrated in FIG. 3,the gel coat is first sprayed at step 37 into a boat-hull mold andallowed to cure. The structural glass fiber is layered at step 39 in thesame manner as in conventional boat construction. In a preferred aspect,the stringers are then installed at step 41, preferably with the layerof structural glass fiber having been impregnated and the stringershaving been impregnated with a setting resin. At step 47 the installedstringers are then cured along with the structural layer, and at step 49the boat hull is then removed from the mold.

In an alternative implementation, the structural glass fiber isinitially cured at a step 43 and thereafter at step 45 a light layer ofchopped glass fiber is added at the locations where the stringers are tobe installed. In this alternative, the light layer of chopped glassfiber is impregnated with resin and the stringers are impregnated withresin and then installed. The entire system is then allowed to cure. Theprocess follows thereafter the same as with the initially describedprocess with reference to FIG. 3.

A number of advantages result in construction in accordance with theabove-described system. First, as illustrated in FIG. 4, it will beappreciated that a stringer 51 in accordance with the structure of theinvention can be made with rounded corners 53 which results in reducedstress on the cured glass woven fiber 13 as implemented in use in boatconstruction because resin impregnated cured glass fiber generallyweakens at sharp corners. The appropriate shape of a mold in making thestructure 11 of the invention can avoid such sharp corners. Optionally,to provide increased strength and toughness to the urethane foam 21, anadditional layer 15a of non-woven fabric can be attached to the lid ofthe mold to ensure that the fabric becomes secured to the foam at aregion wherein the reinforcing fabric 13, i.e., glass fiber, is notrequired. As noted previously, the use of the fabric layer interfacedwith the foam 21 provides increased structural properties.

A lower strength, alternative embodiment of a composite structure 90 isshown in FIG. 5, wherein only the fabric layer 15 is utilized, withoutthe reinforcing layer 13, as in the bottom of the stringer shown in FIG.4. In this embodiment, and in accordance with an inventive arrangement,a predetermined amount of a self-expanding, self-curable, uncuredstructural foam is dispensed into the cavity 21. The foam expands andcures in the cavity at a molding pressure determined by thepredetermined amount of the foam and thereby attaches itself to thefabric layer to form the composite structure. The molding pressurecauses the expanding foam to substantially fill only interstices 19 ofan inner portion 91 of the fabric layer, without substantiallypenetrating an outer portion 92 of the fabric layer. An illustrativeborder between the inner and outer portions is represented by a jaggeddashed line 93. After freeing the cured composite structure from theconstraint of the arranging step, the outer portion 92 of the fabriclayer of the composite structure is substantially completely saturablewith a curable material for lamination to another structure in asubsequent processing step. Such other structure may include, forexample, spas, baths, Jacuzzis, shower enclosures and the like. Thecomposite structure in accordance with this inventive arrangement isespecially adapted for use in lower stress environments than suchcomposite structure technology has generally been applied to in thepast. Significantly, however, it can be adapted for use in applicationsrequiring higher strength. For example, if the composite structure isused as a structural reinforcing component, additional layers ofreinforcing fabric may be laminated over the basic composite structureto enhance its strength.

While the invention has been described with reference to its use asstringers or bulkheads for boats, it will be appreciated by those ofordinary skill in the art that other uses can be made of the compositestructures in accordance with the invention. For example, suchstructures can be used as reinforcing elements for automobile body partssuch as hood and trunk lids, bumper beams and structural framing.Likewise, another use can be in architectural applications where thestructures could be integrated into panel fabrication processes asstructural ribs or stiffeners.

Although particular preferred embodiments of the invention have beendisclosed in detail for illustrative purposes, it will be recognizedthat variations or modifications of the disclosed invention, includingthe use of additional layers of fabric, alternative types ofself-expanding foam, and various alternative adhesive or syntheticresins, lie within the scope of the present invention.

What is claimed is:
 1. A method for making a composite structure,comprising the steps of:arranging a single fabric layer in aconfiguration constrained against outward movement and defining a cavitybetween opposing surfaces of said fabric layer; dispensing apredetermined amount of a self-expanding, self-curable, uncuredstructural foam into said cavity, said foam expanding and curing in saidcavity at a molding pressure determined by said predetermined amount ofsaid foam and thereby attaching itself to said fabric layer to form saidcomposite structure, said molding pressure causing said expanding foamto substantially fill only interstices of an inner portion of saidfabric layer, without substantially penetrating an outer portion of saidfabric layer; and, freeing said cured composite structure from saidconstraint of said arranging step, said outer portion of said fabriclayer of said composite structure being thereafter substantiallycompletely saturable with a curable material for lamination to anotherstructure in a subsequent processing step.
 2. The method of claim 1,further comprising the step of:positioning said cured compositestructure adjacent to a second composite structure so that the outerportion of said fabric layer of said cured composite structure is incontact with said second composite structure; saturating with a curableresin said outer portion of said fabric layer of said cured compositestructure and an outer portion of said second composite structure andpermitting said resin to cure.
 3. The method of claim 1, comprising thestep of arranging a non-woven fabric composed of continuousthermoplastic fiber needle--punched together as a felt-like fiber layerin said configuration constrained against outward movement and definingsaid cavity.
 4. The method of claim 3, comprising the step of selectingsaid non-woven fabric layer from one of: a polyester staple mat; a glassfiber mat; a organic fiber mat; and, an inorganic fiber mat.
 5. Themethod of claim 1, comprising the step of dispensing a predeterminedamount of a two-part, self-expanding, self-curing urethane foam.
 6. Themethod of claim 1, comprising the step of experimentally determining anamount of said dispensed foam to be sufficient to cause said foam topenetrate into interstices of said inner portion of said fabric layer assaid foam expands and cures, without penetrating into said outer portionof said fabric layer.